This invention relates to the field of interferometry, and particularly to scanning interferometers intended for use in infrared spectrometry. More specifically, its primary focus is on a major improvement in Michelson interferometers intended for use in infrared Fourier transform spectroscopy. The present interferometry invention makes possible the use of the Fourier transform approach for a wide variety of practical tasks, including stack monitoring, medical gas analysis, liquid and gaseous process control, and the analysis of gas chromatography fractions.
The Michelson interferometer is discussed in detail in the book "Chemical Infrared Fourier Transform Spectroscopy" by Griffiths, published by John Wiley & Sons. The following is quoted from Pages 9 to 10, referring to a figure shown therein:
"The Michelson interferometer is a device that can split a beam of radiation into two paths and then recombine them so that the intensity variations of the exit beam can be measured by a detector as a function of path difference. The simplest form of the interferometer is shown in FIG. 1.1. It consists of two mutually perpendicular plane mirrors, one of which can move along the axis shown. The movable mirror is either moved at a constant velocity or is held at equidistant points for fixed short time periods and rapidly stepped between these points. Between the fixed mirror and the movable mirror is a beamsplitter, where a beam of radiation from an external source can be partially reflected to the fixed mirror (at point F) and partially transmitted to the movable mirror (at point M). After each beam has been reflected back to the beamsplitter, they are again partially reflected and partially transmitted. Thus, a portion of the beams which have traveled in the path of both the fixed and movable mirrors reach the detector, while portions of each beam also travel back toward the source."
If the two paths, or arms, of the interferometer have equal lengths, and the mirrors are properly oriented relative to the beamsplitter, the beams propagated toward the detector will combine constructively and a maximum signal will be obtained at the detector. The signal obtained with nonequal path lengths depends in a complex way on the spectral nature of the radiation. For example, monochromatic radiation, such as that obtained from a single wavelength laser, will give rise to a sine wave of amplitude versus position, wherein the displacement required to trace out one cycle is equal to one half of the radiation wavelength. Radiation which is not monochromatic but is still limited to a specific band of wavelengths will trace out a "damped" sine wave in which the central peak corresponds to equal path lengths.
The pattern traced out, as the length of one interferometer arm is scanned, is the Fourier transform of the wavelength spectrum. Thus, by using a computer to perform the required mathematical transformation the Michelson interferometer can form the basis of a sophisticated spectrometer. Such a "Fourier Spectrometer" has advantagesj over conventional spectrometers in such areas as speed and sensitivity.
Most of the Michelosn interferometers built to date have been scanned by an actual mechanical displacement of one of the mirrors. This method is extremely expensive and delicate, due to the need to control the mirror position to a fraction of one wavelength of the radiation under study (as short as 2 micrometers for most instruments).
Because of this scanning difficulty in prior art interferometers, their use in spectroscopy has been severely limited. This is a limitation which the present invention is designed to remove.
Reference to certain prior art is necessary, even though its purpose is not related to the problem of scanning movement in the interferometer, but instead to the problem created by the limited light-acceptance angle of the instrument. An article by Ring and Schofield, in the March, 1972, issue of Applied Optics (Page 507), shows a number of interferometer designs in which both a moving wedge of refractive material and a moving mirror are used to effect scanning. In all such cases, the purpose has been to increase the acceptance angle of the instrument (i.e., to broaden its field-of-view). Since the goal of these schemes inherently requires both a moving refractive element and a moving mirror, no savings in cost or adjustment criticality is obtained.
A simple example of a field broadened interferometer is referred to in the Ring and Schofield article as "Mertz's first system." This system uses a pair of wedges, positioned so as to approximate a rectangular parallelepiped. As one of the wedges is displaced in the direction indicated, the thickness of the parallelepiped changes. This gives rise to a change in the effective optical path length, by virtue of the fact that the optical path length in the parallelepiped is d=nt, where t is the thickness, and n is the index of refraction.
In the Mertz system, scanning is accomplished by simultaneously moving the wedge and the mirror in such a way as to provide the greatest possible angular field of view over the full scan distance. The proper relationship between the velocities of the movements is discussed in the article. The rectangular compensating element shown therein is not essential to system operation, but does provide improved performance by compensating for the average thickness of the wedge elements.
A similar concept is shown in an article by Despain, Brown, Steed and Baker in the Proceedings of the Aspen International Conference on Fourier Spectroscopy, 1970 (note Page 295), wherein a movable wedge and mirror are combined in one element by silvering the rear surface of a wedge-shaped prism.
As mentioned above, the attainment of the maximum field-broadening effect requires both a moving wedge and a moving mirror. However, a very substantial reduction in motion criticality, and hence cost, can be achieved if the mirror remains stationary and only the refractive element is moved. Specifically, the refractive element can be designed so that a large mechanical motion will result in a relatively small change in optical path length, thus reducing the need for critical position tolerances on the moving parts. One design based on this principle has previously been disclosed. (See Barringer U.S. Pat. No. 3,482,919). This design uses a flat refractive plate in one arm, the angular position of which is mechanically oscillated so as to vary the optical path length in the plate. One disadvantage of this approach resides in the fact that the oscillating motion gives rise to a substantial displacement of the optical beam. This places severe restraints on the design and alignment of the reflector in the scanned arm.
An improved cost-reduced interferometer is the "wedge plate" design described to me by Asron Kassel, a consultant, in January, 1976. This design is functionally identical to Mertz's first system, with the significant exception that both mirrors are stationary. It thus provides reduced motion criticality without introducing as much beam displacement during scanning as the oscillating plate design of Barringer. It has the disadvantage (common to the Mertz system) of requiring transmission through at least four refractive surfaces for each direction of light propagation in the scanned arm. If the transmission at each surface is T, the net transmission will be T.sup.8. This can lead to a significant reduction in performance, since it is impossible to achieve a low reflection loss over the full optical band of usual interest. For example, if T=0.7, the net transmission will be (0.7).sup.8 =0.057. On the other hand, if the number of refractive surfaces can be reduced from four to two, the transmission in this example will be increased to (0.7).sup.4 =0.24.
Since there is a significant relationship between the present invention and the use of retro-reflectors (instead of flat mirrors), this background discussion should also acknowledge that retro-reflectors are not novel per se. Such devices are shown in Diehr U.S. Pat. No. 3,419,331 and in Hubbard U.S. Pat. No. 3,409,375. In both of these patents, however, it is necessary that a retro-reflector be moved in order to effect scanning. Because of their bulk, the movable retro-reflectors add to the difficulty of obtaining reliably-controlled scanning motion.
In order to complete the listing of publications in this field which have come to my attention as a result of novelty searches, the following are noted: Girard U.S. Pat. No. 3,684,379; Barr et al U.S. Pat. No. 3,217,591; Hubbard U.S. Pat. No. 3,409,375; Mertz U.S. Pat. No. 3,469,923; Girard U.S. Pat. No. 3,432,238; and Mertz U.S. Pat. No. 3,246,557.